It is known in the prior art to determine the dose or the dose rate in order to control the voltage of the x-ray tube, of the x-ray tube current, of the exposure duration and/or in order to control a prefiltering from an initially recorded x-ray image. For this purpose, an image segment (dominant) is selected from the initially recorded x-ray image and the data assigned to this image segment is used to determine an actual value for the dose or dose rate. This data relates to the totality of the data values recorded by the image elements assigned to the dominant. To control the dose or dose rate when recording a further x-ray image, the determined actual value is compared with a predetermined target value, and a corresponding change in the control variables takes place. In the case of the exposure time this is a proportional variable. In other words, the exposure time is doubled for instance, if the actual value is only half as great as the target value.
The object of an operator has hitherto been to select the dominant as an image segment. The dominant should comprise typical image segments and the dose or dose rate for the subsequent x-ray recording is optimized to this dominant. An average dose data value from the image elements of the dominant has hitherto been used as the actual value for the dose or dose rate.
In particular, if the dominant of the initially recorded x-ray image comprises regions of direct radiation, i.e. regions in which the radiation emitted from the x-ray tube directly hits the detector, without being absorbed to a significant degree by the object of the image, the average dose of the x-rays received by the image elements of the dominant increases. If an actual value is determined from this increased dose, a control can result in the actual value having to be reduced to a target value. Consequently, the subsequent image is less significantly exposed. This results in a deterioration of the image quality, particularly in significantly absorbent regions.
A method for determining a characteristic for the dose is known from DE 103 11 627 A1, in which the median of the dose data values from a region of interest is used as the characteristic. The median is a value, above and below which half of the data values lie respectively, with said median generally differing from a simple average value.
The treating doctor has thus hitherto been in favor of aligning the dominant to the significantly absorbing regions so as to achieve the best possible contrast. Dominants featuring an image region, in which the direct radiation is significant, generally result in a dose which is too low in the next recorded x-ray image.